Project Summary: The long term objective of the proposed research is to delineate the mechanisms underlying reinforcement learning in the striatum: specifically, the essential second messenger pathways and neuromodulatory interactions mediating synaptic plasticity and plasticity of intrinsic excitability of striatal neurons. This research is significant because reinforcement learning may play a role in alcohol addiction and relapse. As a first step toward achieving this goal, the proposed research evaluates the sensitivity of synaptic plasticity, and underlying second messenger pathways, to the temporal interval between cortical and dopaminergic inputs to striatal neurons spiny projection neurons. Aim 1: Evaluate the hypothesis that the calcium elevation in response to supra-threshold cortical stimulation is enhanced when it is followed by nigral (dopamine) stimulation. Aim 2: Evaluate the hypothesis that long term potentiation occurs only when supra-threshold cortical stimulation is followed by nigral (dopamine) stimulation. Both aims are evaluated using combined electrophysiological and computational modeling approaches. Computationally efficient computer algorithms for stochastic diffusion are developed to allow simulation of second messenger pathways in a multitude of spines on the dendritic tree of an entire neuron. The new algorithm is used to develop a spiny projection neuron model that includes not only in vivo like synaptic inputs, but also the spatio-temporal dynamics of calcium and second messengers activated by dopamine. Experiments are performed on cortex-substantia nigra-striatum organotypic cultures because this preparation has intact, functional cortical glutamatergic and nigral dopaminergic synaptic inputs, producing spontaneous in-vivo like activity in striatal neurons. For each of the specific aims, control experiments are used to adjust model parameters, then model simulations are performed to test the hypothesis, and after that, the model predictions are tested experimentally using the same stimulation protocols. Relevance: The proposed research may contribute to our understanding of alcohol addiction, because it investigates brain structures that respond to both alcohol and behaviors associated with prior alcohol use. Thus, the results of this research may help in the development of treatments for alcohol addiction.